1. Field of the Invention
The present invention relates to an optical film applicable to a flat display.
2. Description of the Prior Art
Since liquid crystal display (LCD) panels cannot emit light, backlight modules serving as brightness sources become an important element for liquid crystal displays (LCDs), and are also quite important for enhancing display brightness. In the industry, there are many attempts to enhance the brightness of LCD panels, for example, by increasing the number of lamps. However, the increase of the number of lamps will easily result in an excessive heat accumulation in LCDs, and thus affect the service life and quality of other elements. Meanwhile, due to large power consumption, the demands associated with many information products that use batteries to provide power when not being connected to power line cannot be fulfilled. Currently, various optical films are used in backlight modules to enhance the brightness of LCD panels and maximize the efficiency of the light sources, without altering any element design or consuming additional energy. This approach has become the most economical and convenient solution.
The light-condensing film, also referred to as brightness enhancing film or prism film in this field, has micro strip-shaped prism structures formed by curing a special acrylic resin with energetic UV light on a polyester optical film having a thickness of merely 50-200 μm, where the micro strip-shaped prism structures are parallel to each other. The main function of the brightness enhancing film involves gathering the scattered light rays coming from the lightguide plate in all directions by means of refraction and internal total reflection, and converging the light rays in the on-axis direction of about ±35 degrees to enhance the luminance of the LCDs. FIG. 1 is a schematic diagram of a conventional brightness enhancing film. As shown in FIG. 1, a plurality of micro strip-shaped prism structures (2) is arranged on the substrate (1), and the strip-shaped prism structures are parallel to each other. Each prism structure is composed of two slant surfaces, where the two slant surfaces meet at the top of the prism to form a pit (3), and every two prisms meet to form a groove (4). The brightness enhancing films disclosed in the prior art have regular strip-shaped structures having a fixed width, such that an optical interference caused by the light rays refracted or reflected by other films of the displays or by other light rays refracted or reflected by the brightness enhancing film occurs, thereby resulting in Moiré or Newton ring in appearance.
Currently, there are many published patent documents that are related to brightness enhancing films, the improvements and fabrication methods thereof, and methods of applying the same to backlight modules. The prior art has disclosed methods to improve the brightness enhancing films. For example, U.S. Pat. No. 5,919,551 discloses using two sets of prism structures with different pit distance (i.e., the distance between two adjacent pits) to prepare a brightness enhancing film. In U.S. Pat. No. 5,771,328, a method of using two sets of prism structures with different pit heights to prepare a brightness enhancing film is disclosed, in which the pit heights refer to the distance from the pit to a reference surface (for example, the upper surface of the substrate). In the prism structures used in U.S. Pat. No. 5,919,551 and 5,771,328, a single prism structure has a single pit height. U.S. Pat. No. 6,354,709 discloses a method of using a single prism structure with different pit heights (i.e., the pit line of the prism is not a straight line) to prepare a brightness enhancing film. FIG. 2 is a schematic diagram of a brightness enhancing film of U.S. Pat. No. 6,354,709, in which a plurality of micro prism structures (6) is arranged on the substrate (5). These prism structures are parallel to each other, and a single prism structure has different pit heights at different lengths. However, even though the pit distances or pit heights of conventional brightness enhancing films are changed, the brightness enhancing structures are still regular; that is, the prisms are parallel to each other (i.e., having parallel pits or parallel grooves). Therefore, the optical interference cannot be alleviated effectively. A solution proposed in the industry is to dispose a piece of diffusion film above the brightness enhancing film, which however suffers from the disadvantages associated with increased costs and complex backlight modules. Moreover, since the conventional brightness enhancing film has a regular brightness enhancing structure, when the brightness enhancing structure is damaged, for example, the apex angle of the prism structure is dented, the light gathering effect will be adversely affected or dark spots occur on the panel. Therefore, it is necessary for the brightness enhancing films to have a high hardness and undamaged brightness enhancing structure, which however, results in an increase on the preparation cost.
The present invention provides an optical film to eliminate the above disadvantages. The optical film of the present invention has non-parallel, adjacent columnar structures, and can gather the light rays and reduce the optical interference. Additionally, as compared with the prior art, the optical film of the present invention can be produced at a lower cost due to an irregular brightness enhancing structure and a less strict processing requirement.